1. Field of the Invention
This invention relates to the recovery and reuse of volatile organic compounds and inert pad gases. More specifically, the invention provides an apparatus and process for recovering and reusing volatile organic compounds and inert pad gases when such mixtures of gases are discharged from marine vessels during loading.
2. Description of the Related Art
The release of volatile organic compounds (VOCs), especially hydrocarbons, into the atmosphere during loading of marine vessel storage tanks long plagued the petroleum industry. Air or inert gases such as carbon dioxide or nitrogen were generally added to marine vessels and tanks upon unloading to fill the space created by pumping out liquid cargo. These gases are referred to as pad gases. Later, as liquid cargo was loaded, the pad gases were displaced by the liquid and had to be vented. The pad gases and any contained VOCs were then lost.
Several different methods were employed to control the emission of VOCs to the atmosphere. Applicant is named as co-inventor of a U.S. Patent in this area, U.S. Pat. No. 5,050,603 entitled, "Mobile Vapor Recovery and Vapor Scavenging Unit."
The methods employed to control these VOC emissions could be divided into three categories: (1) closed loading of tank vessels, more properly termed vapor balancing; (2) incineration; and (3) recovery processes.
Closed loading of tank vessels necessitated loading with all the hatches and ports closed. This was contrary to most barge practice but was routine on most large tank ships. The term "closed loading" did not necessarily imply the capture of vapors, rather, as a tank was being filled, the vapor in the free space above the level of the liquid being loaded was displaced upward into a pipeline that returned the vapor to the free space of the tank being emptied. Thus, the vapor was in effect recycled from the tank being filled to the tank being emptied.
Combustion or incineration processes could be more than 98% efficient if operated properly. They could perform reliably as the sole hydrocarbon control process but were more reliable as polishing units. Polishing units were secondary VOC removal systems that removed lower concentrations of VOC's after another, primary removal system had removed the majority of the VOC's. The primary drawbacks with these processes were that they did not recover the hydrocarbon product and they were a source of air pollution. The value of this incinerated hydrocarbon could be significant when crude or gasoline was being shipped. Furthermore, combustion devices could be relatively unsafe because they were potential sources of fire and explosion caused by the flammable VOCs and hydrocarbon products. The incineration process also produced NO.sub.x that contributed to smog. Thus, incineration was to an extent a self-defeating method since it contributed to the very ill that was being sought to be eliminated.
Vapor recovery processes could be divided into three types: (1) lean oil absorption; (2) refrigeration at atmospheric or higher pressures reached by compressing the pad gases; and (3) carbon bed absorption. Lean oil absorbers operated at pressures of 100 to 200 psia were very efficient at recovering hydrocarbons from rich streams but were less efficient at removing hydrocarbons from streams that contain little hydrocarbon. Typically, an absorber could remove up to about 95% of the ethane and heavier fraction of the vaporous hydrocarbon content of a feed stream by pressure increase and temperature decrease. At temperatures below 60.degree. F., hydrate formation caused freeze-up problems. If the system was under pressure, water could also freeze at temperatures above 32.degree. F. Antifreeze could be used to lower the liquid hydrocarbon freezing point but this added to operating costs. The absorption process could only reduce a vapor stream's hydrocarbon content to 1-3% (volume) of the initial ethane and heavier fraction economically. Thus, the absorber off gas had to be routed to a polishing flare or incinerator.
The direct refrigeration system removed hydrocarbons by cooling and condensing the vapors through a series of low temperature heat exchangers. This process had the advantage that very low temperatures were possible so that up to 99% of a stream's hydrocarbon content could be removed. However, in order to achieve this high proportion of hydrocarbon reduction, temperatures below 60.degree. F. were required and at these temperatures hydrates formed and plugged the exchanger surfaces and lines. This could be avoided by the injection of ethylene glycol or other antifreezes. Direct refrigeration units that employed vapor compression and expansion with regenerative heat exchange against very cold expander discharge refrigerants, were sometimes used. However, even the best of these could not remove ethane and heavier hydrocarbons to the very low levels required by regulatory authorities, i.e., two pounds of hydrocarbon vapor emitted per 1000 barrels loaded.
An apparent solution was to incinerate this stream in a flare, however, the use of such flares was a safety hazard and were unacceptable to the Coast Guard authorities for use on board a ship. Moreover, flares produced NO.sub.x and were to that extent counterproductive since NO.sub.x contributed to smog. Further, the direct refrigeration unit exit stream was so lean that hydrocarbons would have to be added to enrich it to enable combustion. This was a waste of product that was costly to recover in the direct refrigeration process.
Carbon bed absorbers used activated carbon or a similar absorptive material to absorb hydrocarbons selectively. After the absorptive capacity of the carbon was used up, the hydrocarbon would "break through" and appear in increasing amounts in the exiting vapor stream. Generally, the spent carbon would be disposed of. However, if the volume of spent carbon was large enough, a regeneration system to recover the carbon could be cost effective. The best approach was to use a vacuum to desorb the hydrocarbon from the carbon. As an alternative, the hydrocarbon could be steam stripped from the carbon but this generated an oily waste water stream that had to be disposed of. Carbon beds did not do an efficient job of recovering light ends such as ethane and propane. For use in marine applications, carbon beds needed to be very large to handle the high flow rates and hydrocarbon loadings generated.
These methods that were employed to control the emission of VOCs did not allow complete recovery of the VOCs for reuse and recovery of the inert gases for reuse and thus did not provide a totally satisfactory solution to the problem.